1. Field of the Invention
The present invention relates to a remote data reading and transmission system suited to be used for collecting periodically data as sampled simultaneously by a plurality of data sources distributed in a system to be controlled, with the aid of a common optical fiber.
2. Description of the Prior Art
Among various types of information required for effecting control in an electric power system such as a power plant, essential elements will be information relative to an instantaneous value and an effective value of currents and voltages and ON-OFF state of circuit breakers. On the basis of such information, protection, regulation and other controls are made for the power system including a power station, substations, power transmission lines or the like equipments.
In a hitherto known control system for a power system, the information about currents and voltages has been derived in the form of analog quantities scaled down to the order of about 1 ampere and 100 volts. The derived information or data signals are collected, transmitted and processed individually in dependence on its utilization such as for protection of equipment, control of power supplies and demand, open and close operations of circuit breakers and line switches, and supervisory control.
FIG. 1 illustrates a hitherto known protective relaying system for a substation in a power system. The substation includes duplicated buses B1, B2, transformers T1, T2 and two power transmission lines L.sub.1, L.sub.2. In FIG. 1, the broken line represents a zone PA to be protected. Several circuit breakers CB1, . . . , CB6 are installed in the boundary point of the protective zone. Current transformers CT1, . . . , CT6 are installed in adjacency to the respective circuit breakers. Each of the current transformers is connected through individual wiring to the bus protective relaying system BPRD for protection of the buses B1 and B2. In other words, local information (current signals) obtained from the various current transformers is fed to the protective relaying system through the respective individual conductors. Further, since the information or data for protection of power transmission lines, buses, or transformers as well as information required for the purposes other than protection such as for control or regulations, various operations for maintenance or the like are supplied to a central control station through respective different conductors even in the case where some of the information is derived from the same measuring location, the number of such pieces of wiring or conductors for collecting the information will amount to a very large amount for the whole of the substation. Such a case is not shown in the drawings. Accordingly, if the existing individually separated wiring system is continued to be adopted for the transmission of various pieces of information in the power system which itself is going to be become much larger and more complicated, the information transmitting wiring will also become inevitably more complicated, thereby making the maintenance more difficult. Besides, a serious problem will arise in respect of available space for expansion of the power system.
In view of the situation described above, there has been a tendency to adopt digital techniques in control systems for power systems with a view to facilitating multiplex transmission of information. For example, it is possible transmit to data for three phases of a power transmission line to a central control on a time division multiplex base, after the data sampled at the local information source has been converted into digital signals by an A-D converter in a terminal apparatus provided at or in the vicinity of such information source. However, two major difficulties described below remain to be solved in conjunction with such time division multiplex transmission of digital sampled data:
(1) In the case where the functions of the bus protection relaying system such as described above are to be executed by a digital processor such as a digital computer, different sampling times among the data would require a correcting operation for establishing correct correlation among the sampling times, which implies that a lot of time will be consumed for such correcting operation, whereby indispensable requirements such as a high speed response and a high accuracy for fault detection can not be satisfied. Accordingly, in order to reduce the burden or load imposed on the processor, it is necessary to effect the data samplings at the same time in a whole system.
(2) Since the data transmission lines for transmitting digital data from terminal equipments at information sources to a master station in which digital data processors are installed will extend frequently in the vicinity of conductors where large currents flow, there arises an undesirable possibility of electromagnetic or electrostatic noise being induced in the data transmission line, which of course must be overcome.